Role of light in human skin color variation

Abstract

The major source of color in human skin derives from the presence within the epidermis of specialized melanin-bearing organelles, the melanosomes. Tanning of human skin on exposure to ultraviolet light results from increased amounts of melanin within the epidermis. Melanosomes synthesized by melanocytes are acquired by keratinocytes and transported within them to the epidermal surface. In some cases, the melanosomes are catobolized en route. New information indicates that the multicellular epidermal melanin unit (melanocyte and associated pool of keratinocytes) rather than the melanocyte alone is the focal point for the control of melanin metabolism within mammalian epidermis. Gross human skin color derives from the visual impact of the summed melanin pigmentation of the many epidermal melanin units. In theory, constitutive skin color in man designates the genetically-determined levels of melanin pigmentation developed in the absence of exposure to solar radiation or other environmental influences; facultative skin color or "tan" characterizes the increases in melanin pigmentation above the constitutive level induced by ultraviolet light. The details of genetic regulation of pigment metabolism within the epidermal melanin units are being clarified. In some mammals at least, the function of epidermal melanin units is significantly influenced by hormones which may be regulated by radiations received through the eyes. Based on an evolutionary history of the human family which exceeds ten million years, it is proposed that melanin pigmentation may have played a number of roles in human adaptions to changing biologic and physical environments.

Variation of melanoma incidence with latitude in North America and Europe

“…This would suggest that white skin carries a selective advantage in northern latitudes. Quevedo et al (1975) have reviewed the theories which have been advanced to explain this, and concluded that the most attractive was that concerning the synthesis of vitamin D. This vitamin is rare in most foods but can be synthesized in the skin under the action of sunlight. In northerly latitudes a greater proportion of the available sunlight would need to be absorbed to produce sufficient vitamin D. There must however be a compromise between a skin which is light enough to absorb enough UV and one which is pigmented enough to protect against sunburn, solar degeneration and skin cancer.…”

“…The incidence among darkskinned races is commonly low (Oettle, 1]966;Camain et al, 1972) and a clear relationship between the density of skin pigmentation and the incidence of melanoma on the exposed body sites has been demonstrated (Crombie, 1979). Melanin pigmentation is thought to afford protection by absorbing the UV (Quevedo et al, 1975) so that the degree of protection would be expected to depend on the concentration of pigment.…”

Genetics of dark skin in mice

“…1b). The latter class is particularly interesting from the perspective of human genetics, as normal variation in human skin color is mediated by changes in melanocyte activity rather than melanocyte number (Toda et al 1972;Quevedo et al 1975;Quevedo and Holstein 1998). Beside rare mutations that cause oculocutaneous albinism, the only human skin color gene identified to date is the melanocortin 1 receptor, in which loss-of-function variants cause fair skin and red hair (Rees et al 1999;Sturm et al 2001).…”

Higher milk fat content is associated with higher 25-hydroxyvitamin D concentration in early childhood

“…These included: children's age, sex (Hagenau et al 2009;Lagunova et al 2009;Rockell et al 2005), skin pigmentation (Maguire et al 2013), BMI z-score (zBMI), daily vitamin D supplementation, daily volume of milk consumed, date of blood collection, non-cow's milk consumption, and median neighbourhood family income (Statistics Canada 2014). The Fitzpatrick scale was used to assess skin pigmentation, an acceptable method for skin pigmentation quantification used in dermatological research (Fitzpatrick 1988;Quevedo et al 1975).…”